The ability of rotator cuff tendons to heal back to bone following injury is limited and incomplete healing of surgically repaired rotator cuff tears in humans has been reported in 20-70% of cases. Many factors outside of the surgeon's control contribute to the limited healing potential including patient age, tear size, and time from injury to repair. However, two important factors that are within the surgeon's control are surgical repair technique and post-operative rehabilitation protocol. Much research has been done on surgical repair technique including studies related to repair approach, strength of suturing methods, and security of anchoring techniques. Surprisingly, while post-operative rehabilitation protocols are generally understood to have a great influence on the fundamental mechanisms of the healing process and on the resulting function, very little data is available on this topic to guide the clinician. As a result, most post-operative protocols are based on anecdotal experiences of particular clinicians. In a recent study in our laboratory, we developed an animal model in which the healing of the rotator cuff tendon to bone insertion site could be carefully evaluated. Using this model, the overall objective of the current study is to investigate the healing process of the rotator cuff tendon to bone insertion site subject to immobilization and remobilization. Biomechanical and structural properties will be measured to evaluate the effect of the post-operative protocol on healing. In addition, histologic and molecular biologic assessments will be performed to begin to elucidate the mechanisms through which biomechanical and structural properties were altered. The Specific Aims are: (1) following tendon to bone insertion site injury and repair, vary the immobilization period for a given set of remobilization period. (2) Following tendon to bone insertion site injury and repair, vary the remobilization period for a given set of immobilization period. We believe that a long remobilization period, following a sufficient immobilization period, will allow adequate integration of tendon fibers into bone and expression of appropriate ECM proteins to re-form the insertion site resulting in an insertion site with improved mechanical properties and more aligned collagen fibers. [unreadable] [unreadable]